The present invention relates to a system and method for automatically controlling a vehicle speed to a desired cruising speed. The present invention is applicable to the system and method for automatically controlling the vehicle speed in which a running speed of a vehicle is automatically controlled to coincide with a desired set cruising speed.
Similar systems for automatically controlling vehicle speeds are exemplified by a Japanese Patent Application First Publication (Tokkai) No. sho 59-8023 published on Jan. 17, 1984.
In the above-identified Japanese patent application publication, the automatic vehicle speed controlling system includes a vehicle speed sensor which senses the actual vehicle speed and outputs an operation data in proportion to the actual vehicle speed, a command switch group for operatively outputting a cruise command signal, a memory for storing the operation data on the vehicle speed in response to an on-and-off operation of the command switch group, an actuator for actuating a throttle valve of a vehicular engine, and a control unit for transmitting a command to the actuator according to a difference between the actual vehicle speed and stored vehicle speed.
In the automatic vehicle speed controlling system, a set switch installed in the command switch group is operated to turn ON and thereafter to turn OFF so that the vehicle speed at the time of the OFF operation is stored into the memory. The control unit transmits the command to the actuator according to the difference between the stored vehicle speed and actual vehicle speed. At this time, the actuator is operated to actuate the throttle valve so that the vehicle speed is adjusted toward the stored vehicle speed to run the vehicle at the stored cruising speed. In addition, the actuator is operated to actuate the throttle valve so that the vehicle is accelerated in response to an ON operation of an accelerator switch incorporated in the command switch group. Furthermore, in response to the OFF operation of the acceleration switch the vehicle speed is constantly returned at the stored cruising speed at the time of the OFF operation of the accelerator switch. Such automatic vehicle speed controlling systems are convenient for the vehicle run on freeway roads.
Since, however, engine output characteristics, speed reduction ratio in a transmission, and vehicle body weight are individually different as the vehicle models to which the conventional vehicle speed controlling systems are applied are different, an abrupt deceleration duration T.sub.ACC . OFF, during which the actuator is operated so that the vehicle speed is abruptly reduced immediately after the OFF operation of the acceleration switch, needs to be adjusted for each vehicle model on which the above-described system is applied in a case when the vehicle constantly runs at the cruising speed at the time of the OFF operation of the acceleration switch after the acceleration switch is once operated to turn ON. If the adjustment of the abrupt deceleration duration is incomplete, the vehicle speed exhibits an overshooting or undershooting state until the vehicle speed is returned to and settles at the set cruising speed.
Since, in addition, the abrupt deceleration duration T.sub.ACC . OFF is easily affected by a speed range in which the vehicle runs at the cruising speed, and by a road condition (ascending slope, or descending slope) if conditions to set the vehicle speed to the desired cruising speed are different, an optimum abrupt deceleration duration T.sub.ACC . OFF meeting every condition cannot be set.